~ Stuff I Learned About Tech+Hydroponics

Building an LED Grow Lamp – Let There Be Light

I recently finished building a one plant hydro planter (prototype) that includes a built-in air pump and an LED light. My plan is to write some posts documenting what I learned about building an LED lamp in the process. The posts I plan to write include:

Stuff about the quality of light I need to know in order to choose the LEDs. (This post)

Stuff about the quantity of light I need to know.

Design of the prototype LED grow lamp including LED chip choice and other implementation implications.

Testing the prototype planter by tracking the growth of a basil plant from seed to making pesto.

The Goal

The goal of this post is to get a strong feel for how what light quality means to plants and how that affects the choice of LEDs. Specifically, gain a firm understanding of the two light enabled processes: photosynthesis and photomorphogenesis. Up until this point, I had a fair knowledge about how plants use light for photosynthesis. However, I didn’t know much about photomorphogenesis. Folks had kindly (and slowly) tried to explain photomorphogenesis to me…but…the explanations just didn’t stick. I continued to attribute stuff that happened because of photomorphogenesis to photosynthesis. Besides, there are a lot of syllables in the word photomorphogenesis. All those syllables made me think photomorphogenesis was intimidating as a concept to grasp. If it wasn’t, then wouldn’t it have a more approachable/friendly name? Most likely many of you are wondering (perhaps in disbelief?) how I could be so challenged in grasping what you consider simple concepts. If that is the case, this post is a waste of your time!

Lighting for Plant Growth (for Dummies Like Me)

Without light, no plants. Without basil, no pesto. As I mentioned earlier, the two light processes I needed to have a firm grasp of where light quality is important are photosynthesis and photomorphogenesis.

But what is light quality?

Light Quality

As noted in Dr. Erik Runkle’s article Light Quality Defined: “…light quality refers to the spectral distribution of light, or the number of photons of blue, green, red, far red and other portions of the light spectrum emitted from a light source.”

now onto two key light enabled processes that go on in a plant – photosynthesis and photomorphogenesis.

Since I appreciate stuff like Ohm’s law to simply explain important electronics concepts, I’m starting with the equation that simply explains photosynthesis:

I liked this description: Chemically speaking, the inputs to photosynthesis are six carbon atoms, 12 hydrogen atoms and 18 oxygen atoms. Glucose uses six carbon, 12 hydrogen, and six oxygen molecules. Simple math shows 12 leftover oxygen atoms, or six oxygen molecules. Interestingly, and not coincidentally, the process of respiration breaks apart the glucose molecule. Respiration occurs in the cells of nearly all living things…” …respiration releases the energy the plant uses to grow.

“…Blue light (400 to 500 nm)… Chlorophyll in plants highly absorbs blue light that is used for photosynthesis. It also helps regulate the opening of stomata, which are tiny openings in the leaves that regulate the uptake of carbon dioxide…”

“…Green light (500 to 600 nm)…Plants appear green because they reflect and transmit slightly more green light than they do blue or red light…. generally, green light is less efficient at stimulating photosynthesis than blue or red light…”

“…Red light (600 to 700 nm)… Most LED arrays [for plant growth] emit a high percentage (often 75-90 percent) of red light because it is absorbed well by chlorophyll, and the electrical efficiency of red LEDs is high.

Notice under the rationale for red light the mention of electrical efficiency. Looking at a graph from Cree’s XP-E2 data sheet:

the voltage needed to power red LEDs is much less than the voltage needed to power blue or green LEDs. Since P=IV, the cost of electricity (e.g.: in KW/hour) will be less if there is more red LEDs than blue (or green).

Besides photosynthesis, research has shown red and blue wavelengths of light have a big effect on photomorphogenesis.

Photomorphogenesis

Now this just plain out amazes me…plants have light sensors. Who knew? Not me…So many times I have built some sort of prototype that uses a light sensor like this one:

I use it to detect simple stuff. Like how light or dark the light sensor thinks it is. Here is link to an Instructable that gives an example of an Arduino project that uses this type of simple light sensor.

Plants use of their much more sophisticated light sensors to – as explained in this article to “…manipulate the shape and characteristics of a plant, such as leaf size, branching, and stem length.” Saying the same thing in a slightly different way, plants use light sensors to detect the light conditions under which they are growing. The plants use the readings they get from the light sensors to manipulate their size and shape. By manipulating size and shape, the plant can optimize for the light conditions it finds it self under.

Light Sensors Are Detecting Red and Blue Light

Phytochrome and cryptochrome are the pigments found in a plant acting as light sensors. Phytochrome detects red (and far-red) light. Cryptochrome detects blue light. It makes sense there is a close relationship between optimizing the plant’s shape to the light source since light is providing a necessary ingredient to make food. For more info on these pigments, I found this article interesting.

Given a bunch of LEDs mounted on a PCB, what percentage should be red and what percentage should be blue? The stuff about photosynthesis and photomorphogenesis prepares me to make an informed (first) attempt at setting this percentage. The factors I use in deciding the percentage include:

Red light is more efficient for both photosynthesis and photomorphogenesis

photomorphogenesis: from this abstract: “Plants with as little as 10 µmol·m−2·s−1 of B(blue) light were 23% to 50% shorter and had 17% to 50% smaller leaves than plants under only R(red) light.”

Heidi Wollaeger – our teacher for Michigan State’s lighting course and co-author of the peer-reviewed article “Growth and acclimation of impatiens, salvia, petunia, and tomato seedlings to blue and red light.” pointed out to me in an email exchange: “…in a sole-source environment (no sunlight), blue light activates cryptochrome (a photoreceptor in plants) which triggers a change in hormones in the plant which results in reduced stem extension and leaf expansion. You will see that 100% R light (without sunlight) leads to the most elongated plants with the largest leaves. A minimum of 10% B inhibits that excessive stem extension and leaf expansion. What is “ideal” is all a matter of perspective. If you (are) an ornamental plant plug producer and want small compact rooted plants – you will want some B or green in the spectrum. If you a basil grower indoors, you would want those large leaves and might want to have a 100% R.

Blue to Red Ratio for the Prototype

Given I will be growing basil, I will go with 100% Red LEDs

What I’ve Learned After Writing this Post

I have a much better feel for photosynthesis and photomorphogenesis.

I understand what light quality is.

I understand the importance of red and blue light to photosynthesis and photomorphogenesis.

I have made an informed choice on the Blue:Red ratio – 0:100 – for prototyping growing basil.

What’s Next

Now that I have a better feel for light quality (the Blue:Red ratio and what it means for photosynthesis and photomorphogenesis), I need a better understanding of light quantity. Then onto design and implementation of the LED portion of the planter.

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8 thoughts on “Building an LED Grow Lamp – Let There Be Light”

Hi!
very interesting article! I really like it. I was doing research like this, too.

did you hear about chopper light? this might make things even more interesting!
this may boost the efficiency of plant growing using artificial light to the next level!
I did not find much information about this topic.

the idea is to switch off the light source as long as the chlorophyll-complex is busy with the conversion of CO2 + H2O to O2+Sugar. The conversion progress takes some time and during this time, no light is needed for the photosynthesis.

in my assumption this would require a light source that is switched on for ~10µs and is off for about 1ms. This would allow super high pulsed currents for the LEDs and cooler junction temperatures leading to higher efficiency. PLUS: the duty cycle of this setup is only 1% !

Hello Peter, I like the creativity of this approach. I do not have the background/skill set to speak of the efficacy. It would be interesting to read peer reviewed research on this brand/technique. For me, the flashing is extremely disorienting. Also, I am so far behind in my understanding of how to best use Red/Blue to grow the “best” vegetables…I have a lot of learning to do before I would look into this technique.

Whoa…interesting stuff…for me though – I am not even close to understanding other things like R:B ratio and best fertilizer…If I am lucky, I will reach my quest of growing extremely good tasting vegetables and herbs. The kind that anyone who tastes can’t help but smile. For example, today I am excited to give a tomato I have grown especially for the person that delivers our mail. This person does an excellent job. I realize one tomato isn’t such a big gift, but I figure with an appreciative note and a great tasting tomato a smile as well as a great taste will reach the lips of a dedicated person.